1. Field of the Invention
The invention relates to a device for delivering a liquid additive. Devices of said type are used for example in motor vehicles in order to supply a liquid additive to an exhaust-gas treatment device for the purification of the exhaust gases of an internal combustion engine. The invention is used in particular in exhaust-gas treatment devices in which nitrogen compounds in the exhaust gas react, with the aid of a reducing agent, to form non-hazardous substances. The corresponding exhaust-gas purification method implemented in said exhaust-gas treatment devices is the method of selective catalytic reduction (SCR method).
2. Related Art
Ammonia is normally used as reducing agent for the SCR method. Ammonia is normally supplied to the exhaust-gas treatment device not directly but rather in the form of a reducing agent precursor solution, which can be stored and provided as liquid additive. A widely used reducing agent precursor solution in this context is urea-water solution. For exhaust-gas purification, a 32.5% urea-water solution is available under the trade name AdBlue®.
For the provision of such liquid additives in motor vehicles, it is normally necessary to provide a tank for storing the liquid additive and a device for delivering and/or providing the liquid additive. The device for providing the liquid additive should also comprise a filter, because the liquid additive often contains impurities. Such impurities can damage components of the provision device and/or the exhaust-gas treatment device and must therefore be removed from the liquid additive.
A further problem of liquid additives of this type is that they can freeze at low temperatures. The urea-water solution described above, for example, freezes at −11° C. In the automotive field, such low temperatures arise in particular during long standstill phases in winter. The liquid additive expands as it freezes. The expansion can damage and even destroy lines and ducts in which liquid additive is still situated. It is therefore necessary to implement measures for preventing damage to the provision device by freezing additive.
For effective exhaust-gas purification, it is furthermore necessary for the liquid additive to be supplied to the exhaust-gas treatment device in accurately dosed fashion. It is necessary in particular to prevent air bubbles from passing into the device for delivering liquid additive. Air bubbles influence the delivered amount of liquid additive, wherein the influence often cannot be precisely determined because it is (often) not possible (by sensors) to distinguish whether air bubbles or liquid additive are being delivered.
Furthermore, a tank for storing the liquid additive and a device for providing the liquid additive require additional space and lead to additional weight in a motor vehicle. It is therefore advantageous for a device for delivering liquid additive to be able to empty a tank as completely as possible. Even when the tank has already been almost completely emptied, it should still be ensured that no air bubbles are delivered. The tank could then be designed to be very small, such that the additional weight of the tank is low and the tank has a small space requirement.